The current stable of licensed vaccines in the human and veterinary arenas is generally successful against what are termed “Class One pathogens.” Class One pathogens (such as measles, mumps and rubella viruses) are those pathogens, which, in general: (1) infect or cause the most serious disease in infant, very young children, children, and young adults; (2) carry a relatively stable microbial genome; (3) have a natural history of disease which results in spontaneous recovery; and (4) induce durable memory, associated with polyclonal and multi-epitope antigen recognition.
In contrast, Class Two pathogens, such as, influenza virus, HIV-1, malaria parasites, Mycoplasma, such as those that cause tuberculosis, Trypanosomes, Schistosomes, Leishmania, Anaplasma, Enteroviruses, Astroviruses, Rhinoviruses, Norwalk viruses, toxigenic/pathogenic E. coli, Neisseria, Streptomyces, nontypeable Haemophilus influenza viruses, Hepatitis C virus, cancer cells etc. are characterized by quite opposite features. For example, Class Two pathogens: (1) tend to infect and are transmitted in a significantly extended host age range, with infections occurring and reoccurring from childhood through the geriatric period; (2) exhibit microbial genetic instability in defined regions of their genome (a hallmark of the successful evolution of such pathogens); (3) in some cases, include spontaneous recovery of disease that frequently still leaves the host vulnerable to multiple repeated annual infections and/or the establishment of either a chronic/active or chronic/latent infectious state; (4) induce oligoclonal, early immune responses that are directed to a very limited set of immunodominant epitopes which provide either narrow strain-specific protection, no protection and/or enhanced infection; and (5) cause immune dysregulation following infection or vaccination, e.g. epitope-blocking antibody, atypical primary immune response Ig subclasses, anamnestic cross-reactive recall and inappropriate TH1 and/or TH2 cytokine metabolism.
At the immunologic level, very different etiologic agents can yield diverse pathogenesis and disease outcome as observed, for example, with HIV-1 verses human rhinovirus. Highly successful immune system evading strategies, such as, Deceptive Imprinting, have evolved and are selected and maintained across host and microbial taxa. Thus, the operational failures of the vertebrate immune system, for example, arising from pathogen Deceptive Imprinting, are fundamentally the same whether infected with HIV-1 or with the common cold virus for an average of 2-6 times a year for 60 years.
Although some advances with regard to antigen delivery and expression have improved the immunogenicity of some Class Two microbial pathogens, current vaccine technologies have not readily translated into new, broadly effective and safe, licensed vaccines for use in humans. That may be due, in large part, to a poor understanding of the fundamental laws governing the vertebrate host defense system origin, repertoire development, maintenance, activation, senescence and co-evolution in similar and dissimilar environments.
What is lacking currently in human influenza vaccine development is a composition that induces immunity and protection which is less homotypic and subtype-dependent and would therefore not require the mixing and production of multiple subtypes in the current egg-based technology production scheme year to year. A suitable new product is an influenza recombinant HA or NA subunit vaccine that induces immune responses capable of cross-neutralizing both intra-subtype antigenic variants and hetero-subtypes of influenza virus.
Influenza is a NIAID Category C pathogen and causes 36,000 deaths and 220,000 hospitalizations in the U.S. every year. A respiratory disease, influenza spreads through droplets and/or contaminated fomites from the cough or sneeze of an infected person. Higher risk groups include children and the elderly, and having influenza commonly leads to secondary complications of influenza-related pneumonias, upper respiratory complications (otitis media in children) and other systems diseases (e.g. cardiovascular etc.). Influenza is the source of the worst pandemic in history; the Spanish flu of 1918 caused over 40 million deaths worldwide. In the U.S., the annual direct medical costs (hospitalization, office visits, medication etc.) of influenza are estimated at $4.6 billion. Furthermore, each year, up to 111 million workdays are lost because of influenza with an associated cost to American businesses of more than $7 billion a year in sick days and lost productivity. Total direct and indirect costs (work days lost, school days lost etc.) of a severe influenza epidemic are at least $12 billion per year.
Influenza virus, and when attended by secondary bacterial infections, has long been known to be a cause of excess morbidity and mortality. Complications include pneumonia, bronchitis, congestive heart failure, myocarditis, meningitis, encephalitis and myositis. Some groups of people at high risk for complications are those with chronic pulmonary or cardiovascular disorders, residents of chronic care facilities, including nursing homes, and those persons 85 years and older. (Recommendations of the Advisory Committee on Immunization Practices (ACIP) for Prevention and Control of Influenza. MMWR, 1996, Vol 45; and Thompson et al., JAMA 2003; 289:179-186). The geriatric population in the United Stated has doubled between the years 1976 and 1999, and is expected to rise over the next few years as the post World War II baby boomers age. People in that age bracket are 16 times more likely to die of an influenza-associated disease than are persons aged 65 to 69. Another important contributing factor to the increase of influenza-associated deaths in the 1990's is the predominance of the influenza A (H3N2) virus, a more virulent form of the recently circulating influenza viruses.
Influenza is a single-stranded ribonucleic acid (RNA) virus which mutates rapidly to form new virulent strains. The strains are classified into three groups, influenza A, B and C. The virus is further classified based on two surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), into 15 HA and 9 NA subtypes. Recent whole genome analysis of the human influenza virus sponsored by the NIAID/NIH and collected between 1996-2004 from New York State revealed that despite sharing the same HA, multiple, persistent, phylogenetically distinct lineages co-circulate in the same population resulting in reassortment and the generation of antigenically novel clades. While antigenic variance of HA is still the dominant selective pressure on human influenza A virus evolution, the finding that antigenically novel clades emerge by reassortment among persistent viral lineages rather than via antigenic drift is of major significance for the current dated annual method of influenza vaccine strain selection and production (Holmes et al., PLoS Biol. 2005 3(9):e300).
At the heart of the problem in the annual global virus tracking programs and subsequent “reactionary” vaccine production that ensues, is the issue of antigenic variation. Antigenic variation is an evolved mechanism to ensure rapid sequence variation of specific pathogen gene(s) encoding homologues of an individual protein antigen, usually involving multiple, related gene copies, resulting in a change in the structure of an antigen on the surface of the pathogen. Thus, the host immune system during infection or re-infection is less capable of recognizing the pathogen and must make new antibodies to recognize the changed antigens before the host can continue to combat the disease. As a result, the host cannot stay completely immune to the viral disease. That phenomenon stands as one of the more, if not, most formidable problem challenging modern vaccine development today.
Not surprisingly, the immune response generated after infection or vaccination with all currently licensed vaccines is highly subtype and strain specific. In practice, that means antibodies elicited during natural, experimental infection and vaccination are only capable of neutralizing the homologous virus. The subtype/strain-specific humoral immune response appears to be due to the relative immunodominance of various antigenic sites found on the globular head of the hemagglutinin molecule (Wiley et al., Nature, 1981; 289:373-378). More specifically, the antibody response has been mapped to five major antigenic sites within the globular head of the HA. Of the five HA epitopes (A-E), two sites, A and B, are the most immunodominant and also were associated with the highest amount of amino acid hypervariability, due, in part, to reoccurring point mutations, deletions and occasional introduction of N-linked glycosylation sites, known collectively as the “antigenic drift” of the virus (Cox & Bender, Semin. Virol. 1995; 6:359-70; Busch et al., Sci. 286:1921, 1999; Plotkin & Dushoff, PNAS 100:7152, 2003; and Munoz & Deem, Vaccine 23, 1144, 2005).
Original antigenic sin, first described in 1953 by Francis (Ann. Int. Med., 1953, 399:203) is a primary immune response, that when boosted not by the homologous, but by a cross-reacting vaccine or incoming viral subtype/strain, results in the newly formed antibodies reacting better with the previous antigen than with the incoming antigen.
The loss of immune specificity directed by that aleatory recall poses a real problem for the host immune system to mount equal and potent humoral responses to the changing virus both during an infection and between infections. Thus, it is not surprising that natural infection and vaccination fail to yield a more functional cross-reactive primary and anamnestic immunity as the repertoire development against those less immunogenic epitopes, which may be most conserved and capable of generating cross-strain immunity, are lower on the antigenic hierarchy. The immunologic phenomenon whereby immunodominant epitopes misdirect the immune response away from more conserved and less immunogenic regions on an antigen was initially termed “clonal dominance” (Kohler et al., J Acquir. Immune Defic. Syndr. 1992; 5:1158-68), which later was renamed as “Deceptive Imprinting” (Köhler et al., Immunol. Today 1994 (10):475-8).
The immunologic mechanisms for immunodominance behind deceptive imprinting are not fully understood, and no one mechanism yet fully explains how or why certain epitopes have evolved to be immunoregulatory and immunodominant. The range of immune responses observed in the phenomenon include the induction of highly strain/isolate-specific neutralizing antibody capable of inducing passive protection in experimental animal model-viral challenge systems all the way to the induction of a binding non-protective/non-neutralizing, blocking and even pathogen-enhancing antibody that, in some cases, prevents the host immune system from recognizing nearby adjacent epitopes to interfering with CD4 T cell help. The same decoying of the immune response through immunodominance resulting in a more narrowly focused set of epitopes is observed with T cells of the host helper and cytotoxic cell-mediated immunity (Gzyl et al., Virology 2004; 318(2):493-506; Kiszka et al., J. Virol. 2002 76(9):4222-32; and Goulder et al., J. Virol. 2000; 74(12):5679-90).
Vaccination is the best way to prevent the disease and the current trivalent killed virus and modified live (attenuated) influenza vaccines are developed every year based on world-wide epidemiological surveillance of active viral strains. Both vaccines contain influenza A and influenza B subtypes. The licensed influenza vaccines consist of inactivated whole or chemically split subunit preparations from two influenza A subtypes (H1N1 and H3N2) and one influenza B subtype. Production of influenza vaccines involves the adaptation of the selected variants for high yield in eggs by serial passage or reassortment with other high-yield strains. Selected influenza viruses are grown in chicken eggs, and the influenza virions purified from allantoic fluid. Whole or split virus preparations are then killed by treatment with an inactivating agent, such as formalin. More than 90% of the United States market for the vaccine is served by two companies, Aventis Pasteur with more than 50% market share and Chiron (PowderJect) (U. K.). An intranasal vaccine, FluMist®, was approved and first sold in 2003.
Limitations of the currently available influenza vaccines include:
(1) Reduced efficacy in the elderly. Among the elderly, the rate of protection against illness is lower, especially for those who are institutionalized (Gorse et al., J. Infec. Dis. 190:11-19, 2004). Significant antibody responses to a trivalent subvirion influenza vaccine were observed in less than 30 percent of subjects ≥65 years of age (Powers & Belshe, J. Infec. Dis. 167:584-592, 1993);
(2) Production in eggs. The current manufacturing process is dependent on chicken eggs. Influenza virus strains must replicate well in eggs and a large supply of eggs is required each year. Production is at risk each year because of the need to find a suitable virus combination;
(3) Inability to respond to late appearing and drift strains, such as A/Sydney/5/97 in the late nineties, or to respond to a potential pandemic strain, such as the Hong Kong H5N1 virus that appeared in 1997;
(4) Protection with current whole or split influenza vaccines is short-lived, and effectiveness wanes as genetic changes occur in the epidemic strains of influenza due to antigenic variation. Ideally, the vaccine strains are matched to the influenza virus strains causing disease. Changes can occur in the hemagglutinin of egg-grown influenza virus when compared to primary isolates from infected individuals (Oxford et al., J. Gen. Virol. 72:185-189, 1989; and Rocha et al., J. Gen. Virol. 74:2513-2518, 1993) reducing the potential effectiveness of the vaccine;
(5) The side effect of having the vaccine produced in eggs for those allergic to eggs; and
(6) The current licensed manufacturing system yields one vaccine per chicken egg infected with the influenza virus and the production time is approximately 24 weeks.
Thus, the current licensed influenza vaccines do not: (1) induce antibodies capable of neutralizing the common annually recurring antigenic variants circulating during an epidemic, as well as the sub-type and reassortment viruses; (2) illicit a strong immune response in the elderly; and (3) find wide applicability due to side effects, for example, some vaccines cannot be administered to children.